The present invention relates to an improved air filter assembly and a method of fabricating that assembly. In particular, the present invention is directed to improved high efficiency and ultra high efficiency particulate air filters and assembly process for these types of filters. High efficiency particulate air (HEPA) filters have a particle removal efficiency of at least 99.97% for particulates of 0.3 microns. Ultra high efficiency filters have efficiencies of at least 99.9999% for 0.12 micron particulates.
HEPA filters find a wide variety of applications including filtration systems for laboratory and clean room environments where even the smallest amount of particulate in the air is unacceptable. Other typical applications include the filtration of hazardous materials, such as radioactive or biologically harmful materials, from a contaminated air stream before exhaustion of the air to the atmosphere. The prior art HEPA filters typically are constructed utilizing a pleated element of a suitable material in a generally rectangular configuration. The filter element is housed in a rectangular frame comprising four frame members joined together and in which the edges of the filter element are sealed. The frame members can be of any suitable material and typically may be extruded metal or plastic. Prior art HEPA filters of this type are disclosed in U.S. Pat. Nos. 4,685,944; 4,584,005; and 4,227,953.
The prior art HEPA filters are typically assembled by constructing the frame around the filter element. For example, one of the frame members may be filled with an epoxy or sealant in a liquid or viscous state. The filter element is then placed on the frame member with one edge of the element immersed in the epoxy which is then allowed to hardened. The process is then repeated for the oppositely disposed frame member. Then in a similar fashion the filter element is bonded to the two other frame members and the frame members secured together in some manner.
Prior art HEPA filters fabricated according to conventional processes have experienced leaks between the filter element and the frame. Such leaks commonly occur at the corners where adjacent frame members are joined. It is believed that one cause of these leaks is an uneven distribution of the epoxy caused when the epoxy is displaced as the filter element is inserted in the frame member during the assembly process. It can be appreciated that even a relatively small leak could totally destroy the filter efficiency and render it useless.
The present invention is an improved HEPA filter structure and method of fabricating that structure so as to eliminate leaks and in particular the corner leaks that have developed in the prior art. Moreover, the fabricating method and new structure simplifies assembly. In building the filter frame around the filter element in the prior art the step of inserting the filter element in the viscous epoxy or sealant often caused sealant to be pushed out the ends of the frame members overflowing the end of the frame member intended to be joined to an adjacent frame member. Thus, during assembly this displaced epoxy had to be removed by suitable solvent not only for aesthetic reasons in the appearance of the final filter structure but also to ensure that a proper fit was achieved between adjacent filter members at the corners of the assembly. In the method of the present invention, the necessity for this relatively messy solvent clean up step in the fabrication process is eliminated.